Flat-panel displays are commonly used as computer screens, television screens, and electronic game displays and in a variety of other applications. A typical flat-panel display is capable of displaying a composite image formed by a regular array of picture elements, or pixels. Moreover, the image displayed by a flat-panel display may be either monochrome or color, and the pixels of either type of display may be bi-state or multi-state. Bi-state pixels may assume two possible appearances corresponding to first and second levels of attenuation of light (e.g., on/off, light/dark, attenuated/unattenuated, color-1/color-2, etc.). Multi-state pixels may assume those two appearances, as well as up to an infinite number of further appearances corresponding to intermediate attenuation levels.
At each pixel location, light may be provided by a variety of means. For example, a light-emitting device such as a light-emitting diode or other electroluminescent component may be disposed at each pixel location. Alternatively, a backlight may be provided behind the display, and an optical shutter for selectively transmitting and blocking transmission of light may be located at each pixel location of the display. One example of such an optical-shutter-type flat-panel display is a liquid-crystal display (L.C.D.) having a liquid-crystal cell at each pixel location of the display.
Conventional L.C.D.'s include a liquid-crystal cell at each pixel location and a layer of liquid-crystal molecules disposed within each liquid-crystal cell. The liquid-crystal molecules are normally arranged in a twisted-nematic phase, preferably at an angle of about ninety degrees (or an odd-integer multiple thereof) along the path of transmission of light. However, when an electric field is applied across the liquid-crystal layer, the liquid-crystal molecules align with the electric field so that they are no longer arranged in a twisted-nematic phase. Such displays also include a pair of polarizing filters which are disposed on either side of the liquid-crystal cell and have perpendicular polarization directions.
A light source disposed behind the liquid-crystal display transmits light through the display toward a viewer. Alternatively, ambient light may be reflected through the display toward the viewer. In either case, as light passes through the display at a particular pixel location, the light is plane-polarized by the polarizing filter disposed on the surface at which the light enters the display. The plane-polarized light then encounters the liquid-crystal layer.
If the liquid-crystal molecules at the particular pixel location are arranged in a twisted-nematic phase (i.e., if no electric field is applied across the liquid-crystal layer at the pixel location), then the polarization direction of the plane-polarized light is rotated by the twisted-nematic liquid-crystal molecules as it passes through the liquid-crystal layer. The rotated, plane-polarized light then passes through the second polarizing filter which has a polarization direction parallel to the rotated, plane-polarized light. Consequently, the display appears light at the pixel location. If, on the other hand, the liquid-crystal molecules are aligned with an applied electric field, as described above, the polarization direction of the plane-polarized light is not rotated as the light passes through the liquid-crystal layer. In that case, the unrotated, plane-polarized light is blocked by the second polarizing filter, the polarization direction of which is then perpendicular to the unrotated, plane-polarized light. Consequently, the display appears dark at the pixel location.
A prior-art liquid-crystal display can be of either the active-matrix type or the passive-matrix type. These two types of displays differ in the manner in which the above-described electric field is applied across the liquid-crystal layer.
A passive-matrix display directly applies an electric field at a pixel location by applying a voltage across wires on either side of the liquid-crystal cell at the pixel location. Wires disposed on one side of the display are used to select the row of the pixel location where an electric field is to be applied, while wires disposed on the other side are used to select the column of the pixel Location. The liquid-crystal molecules in the cell at the intersection of two wires across which a voltage is applied remain aligned with the electric field while the voltage is maintained on the wires, whereas the liquid-crystal molecules in other cells remain arranged in a twisted-nematic phase.
In contrast, an active-matrix display includes a thin-film transistor and a capacitor at each pixel location. As in the passive-matrix display, the appearance of each pixel is controlled by an electric field applied thereto. However, in the active-matrix display, the electric field is developed by a charge deposited on the capacitor through the associated transistor at each pixel location.
As fabrication technologies for the thin-film transistors employed in prior-art active-matrix displays have improved, it has become possible to produce larger displays of greater resolution (i.e., the number of pixels per square unit of area) than could be produced previously. This increase in pixel density and number of pixels, however, also increases the probability that any given display will have an unacceptably large number of cell failures. This, in turn, results in an undesirable decrease in the yield of commercially acceptable displays. Further, because the defect density in produced displays increases with the size of the displays, it has not been economically feasible to construct large displays or displays having resolution and image quality characteristics sufficient for applications such as, for example, high-definition television (HDTV).
Passive-matrix displays typically have a relatively low defect density (i.e., few cell failures per display) and thus a relatively high production yield compared to active-matrix displays. Passive-matrix displays do not produce as sharp an image as active-matrix displays, however.
Numerous prior-art attempts have therefore been made to increase the production yield of active-matrix, liquid-crystal, flat-panel displays. Some of these attempts have involved the provision of redundant transistors and connective circuitry connected by laser-fusible links which are burned out of the flat-panel display using a laser if they are defective and which remain in the flat-panel display if they function properly. While it is possible to provide additional or more complex circuitry to operate a display of a given resolution in this manner, such an approach requires that the feature size of circuit components be decreased. This decrease in feature size creates an even greater likelihood of fabrication defects resulting in pixel failures which render a display commercially unacceptable. In addition, operative portions of such displays are often damaged by the very lasers used to burn out inoperative portions thereof. Further, some display failures are so severe that they cannot be overcome by the provision of redundant circuitry and cannot be repaired. The per-unit production cost of these displays is therefore quite high.